A spark plug is mounted to an internal combustion engine (sometimes just referred to as “engine”) and used for ignition of an air-fuel mixture in a combustion chamber of the engine. In general, the spark plug includes an insulator formed with an axial hole, a center electrode inserted in a front side of the axial hole, a terminal electrode inserted in a rear side of the axial hole, a metal shell arranged circumferentially around the insulator and a ground electrode joined to a front end portion of the metal shell so as to define a discharge gap between the center electrode and the ground electrode. With the application of a high voltage to the center electrode, there occurs a spark discharge in the discharge gap between the center and ground electrodes so that the air-fuel mixture can be ignited by the spark discharge.
The insulator is inserted and fixed in the metal shell by crimping an open rear end portion of the metal shell radially inwardly with a step portion of an outer circumferential surface of the insulator retained on a taper portion of an inner circumferential surface of the metal shell. It is common to dispose an annular plate packing between the taper portion of the metal shell and the step portion of the insulator in order to prevent the air-fuel mixture etc. from leaking out to the outside through between the metal shell and the insulator. See, for example, Japanese Laid-Open Patent Publication No. 2005-190762.
In recent high-output engines, there is a tendency that insulators are subjected to larger impacts by vibrations etc. On the other hand, there is a demand to decrease the thickness of insulators for size and diameter reduction of spark plugs. It is therefore difficult to obtain the necessary strength of the insulator to withstand the stress by increasing the thickness of the insulator, even though the stress exerted on the insulator due to impact etc. is increasing. It is particularly likely that, in the insulator, the stress due to impact etc. will be concentrated on a boundary region between the step portion and a leg portion extending toward the front from a front end of the step portion. This results in a greater possibility that cracks occur in such a boundary region between the step portion and the leg portion.
The present invention has been made in view of the above circumstances. It is an advantage of the present invention to provide a spark plug having an insulator capable of achieving good breakage resistance by modifying the shape of the insulator and the state of contact of the insulator with a plate packing without increase in insulator thickness.